Security threads as well as security patches may be mounted on a surface of a security document (e.g., currency or banknote paper) or label either during or post manufacture. Mounting of these devices may be achieved by any number of known techniques including: applying a pressure-sensitive adhesive to the backside of the device and pressing the device to the surface of the document or label; and applying a heat activated adhesive to the backside of the device and applying the device using thermal transfer techniques, to the surface of the document or label.
The production of these security devices and the application of these devices to security documents or labels take place in separate operations. Combining these operations in a continuous roll-to-roll process for producing and transferring these security devices to a final substrate would realize advantages in both speed and precision.
The present invention fulfills this need by providing a continuous roll-to-roll process for producing and transferring security devices in the form of microstructured elements or microstructures from a transfer film to a final substrate.
More specifically, the present invention provides a transfer film for transferring microstructures to a final substrate. The microstructures transferred by the transfer film to a final substrate are single or multi-layer structures that comprise: voids in a substantially planar surface, the voids optionally filled or coated with another material; raised areas in a substantially planar surface; or combinations thereof.
In a first exemplary embodiment, the transfer film comprises a carrier film and one or more thermal release adhesive layers on a surface of the carrier film, wherein the thermal release adhesive layer(s) is made up of a plurality of expandable microspheres and one or more pressure sensitive adhesives.
The term “thermal release adhesive”, as used herein, means an adhesive that decreases its adhesion to a surface when heated to temperatures higher than about 60° C., while the term “expandable microspheres”, as used herein, means polymer microspheres that start expansion and/or foaming when heated to temperatures higher than about 60° C.
Thermal release adhesives (e.g., thermal release tapes) are known in the art and have been used in semiconductor wafer processing and other electronic component manufacturing processes. Suppliers of thermal release products used for electronic applications include Nitto Denko Corporation, 1-2, Shimohozumi 1-chome Ibaraki-shi, Osaka Japan (Nitto Denko), which sells REVALPHA® thermal release adhesive tapes and sheets, and Haeun Chemtec Co., Ltd., Shingil-dong, Danwon-gu, Ansan, Kyungki-do, 425-839, Korea, which sells REXPAN™ heat release film. However, the thickness and cost of these products are prohibitive for anything other than small scale samples, and are not suitable for volume production as described herein.
The term “pressure sensitive adhesive”, as used herein, means an adhesive that needs only minimal pressure to adhere or stick to a surface.
In an exemplary embodiment, the one or more thermal release adhesive layers are prepared from a formulation comprising from about 25 to about 99% by wt. (preferably, from about 75 to about 97% by wt., more preferably, from about 90 to about 96% by wt.) of an energy (e.g., ultraviolet (UV) radiation) curable pressure sensitive adhesive (PSA) formulation, and from about 1 to about 75% by wt. (preferably, from about 3 to about 25% by wt., more preferably, from about 4 to about 10% by wt.) of expandable microspheres.
In this exemplary embodiment, the energy curable PSA formulation generally comprises:                from about 5 to about 95% by wt. (preferably, from about 10 to about 70% by wt., more preferably, from about 30 to about 60% by wt.) of one or more elastomeric oligomers;        from about 1 to about 75% by wt. (preferably, from about 5 to about 60% by wt., more preferably, from about 10 to about 40% by wt.) of one or more tackifying resins;        from about 0.5 to about 75% by wt. (preferably, from about 5 to about 60% by wt., more preferably, from about 20 to about 50% by wt.) of one or more reactive monomeric diluents; and        from about 0.1 to about 15% by wt. (preferably, from about 1 to about 8% by wt., more preferably, from about 3 to about 6% by wt.) of one or more photoinitiators.        
In a second exemplary embodiment, the transfer film comprises a carrier film and one or more cured binder layers. In this embodiment, the microstructures have one or more cured conformal release coating layers on a surface thereof, and are bonded to the transfer film by way of the one or more cured binder layers. Heat is not required to initiate release during transfer of the microstructures.
The present invention further provides a method of using the transfer films described above, which method comprises using the transfer films (a) to transfer the above-described microstructures in a continuous roll-to-roll process to a final substrate, or (b) as manufacturing substrates during production of the microstructures and then to transfer the microstructures in a continuous roll-to-roll process to a final substrate.
Also provided is a process for transferring microstructures to a final substrate. In a first exemplary embodiment, the process comprises subjecting the transfer film first described above in a continuous roll-to-roll process to the following operations: either forming microstructures on, or transferring microstructures to a surface of the thermal release adhesive layer(s) of the transfer film, wherein the microstructures are single or multi-layer structures that comprise: voids in a substantially planar surface, wherein the voids are optionally filled or coated with another material; raised areas in a substantially planar surface; or combinations thereof; and then transferring the microstructures from the transfer film onto a surface of the final substrate.
In a first preferred embodiment, the process comprises: forming the microstructures on a surface of a disposable manufacturing substrate; bringing the formed microstructures into contact with a surface of the transfer film while applying pressure thereto, thereby activating the pressure sensitive adhesive in the thermal release adhesive layer(s) of the transfer film, adhering the microstructures to its surface; stripping away the disposable manufacturing substrate; applying one or more heat and/or pressure activated adhesives to the microstructures on the transfer film; bringing the adhesive coated microstructures on the transfer film into contact with a surface of the final substrate while applying both heat and pressure to the transfer film, thereby causing the microspheres in the thermal release adhesive layer(s) to expand (or foam) and deactivate the pressure sensitive adhesive, allowing transfer of the microstructures onto the surface of the final substrate, while simultaneously activating the adhesive on the microstructures, allowing the microstructures to adhere to the surface of the final substrate.
This embodiment is particularly suited for microstructures having so-called “up/down non-parity” (e.g., refractive optical systems). As will be readily appreciated by those skilled in the art, such structures are intended to be viewed from a top or upper side rather than a bottom or lower side. The inventive process allows the microstructures on the disposable manufacturing substrate to be visually inspected for quality assurance purposes before transferring the microstructures to the transfer film, and further allows the microstructures to be properly positioned in an upright position on a surface of the final substrate.
In a second preferred embodiment, the process comprises: forming the microstructures on a surface of the thermal release adhesive layer(s) of the transfer film; applying one or more heat and/or pressure activated adhesives to the formed microstructures on the transfer film; bringing the adhesive coated microstructures into contact with a surface of the final substrate while applying both heat and pressure to the transfer film, thereby causing the microspheres in the thermal release adhesive layer(s) to expand (or foam) and deactivate the pressure sensitive adhesive, allowing transfer of the microstructures onto the surface of the final substrate, while simultaneously activating the adhesive on the microstructures, allowing the microstructures to adhere to the surface of the final substrate.
This embodiment is particularly suited for microstructures that do not require an upper/lower surface inspection (e.g., conductive circuit elements or structures). Such structures could be symmetrical in cross section and are not necessarily intended to be viewed from a top or upper side rather than a bottom or lower side.
In a second exemplary embodiment, the process is a continuous roll-to-roll process that comprises:                forming the microstructures on a surface of a disposable manufacturing substrate;        applying one or more release coating layers to a surface of the microstructures, the release coating layer(s) conforming to the microstructure surface, and then curing the release coating layer(s);        applying one or more binder layers to a surface of a carrier film and optionally also to the cured release coated surface of the microstructures, and while these surfaces are in contact with each other, curing the binder layer(s);        mechanically removing the disposable manufacturing substrate from the microstructures now bonded to the carrier film; and then        transferring the microstructures from the carrier film onto a surface of the final substrate.        
In a preferred embodiment, the microstructures are transferred from the carrier film onto a surface of the final substrate by: applying one or more heat and/or pressure activated adhesives to the microstructures on the carrier film; bringing the adhesive coated microstructures on the carrier film into contact with a surface of the final substrate; applying both heat and pressure to the carrier film and then lifting the carrier film from the microstructures causing separation between the microstructures and the release coating layer(s), thereby allowing transfer of the microstructures onto the surface of the final substrate, while simultaneously activating the adhesive on the microstructures, thereby allowing the microstructures to adhere to the surface of the final substrate.
Other features and advantages of the invention will be apparent to one of ordinary skill from the following detailed description and accompanying drawings.
Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. All publications, patent applications, patents and other references mentioned herein are incorporated by reference in their entirety. In case of conflict, the present specification, including definitions, will control. In addition, the materials, methods/processes, and examples are illustrative only and not intended to be limiting.